Surveillance system and method utilizing both electrostatic and electromagnetic fields

ABSTRACT

A microwave signal generator projects an electromagnetic wave into a space under surveillance to establish a first field. A pulse or frequency modulated low frequency generator is used to apply a voltage to a discontinuous conductor for establishing a second field, electrostatic in nature, throughout the space. Presence in the space of a miniature passive electromagnetic wave receptor-reradiator in the form of a semiconductive diode connected to a dipole antenna causes the reradiation of the low frequency component modulated on the microwave component as a carrier. The front end of a receiver system is tuned to the microwave frequency and feeds a suitable detector circuit responsive to the low frequency signal. A coincidence circuit energizes an alarm circuit whenever the detected signal coincides with the original modulation envelope being applied to the low frequency generator.

United States Patent Gordon et al.

[ SURVEILLANCE SYSTEM AND METHOD Primary ExaminerGlen R. Swann, lll

UTILIZING BOTH ELECTROSTATIC AND Alromey, Agent, or FirmWatsonLeavenworth ELECTROMAGNETIC FIELDS V Kelwn & Taggart [75] Inventors:Lloyd L. Gordon, Miami; Robert D.

Williamson, Pembroke Pines, both [57] ABSTRACT of A microwave signalgenerator projects an electromag- {731 Assignee: Sensormatic Electronicsnetic wave into a space under surveillance to establish Corporation,Hollywood, Fla. a first field. A pulse or frequency modulated lowfrequency generator is used to apply a voltage to a dis [22] Filed 1972continuous conductor for establishing a second field, [2 l] App]. No.:279,097 electrostatic in nature, throughout the space. Presence in thespace of a miniature passive electromagnetic wave receptor-reradiator inthe form of a semiconduc- Z 'i 340/258 %g;3 ?55 tive diode connected toa dipole antenna causes the 'l 258 C 408' reradiation of the lowfrequency component modul 0 can 343 LC 8 lated on the microwavecomponent as a carrier. The front end of a receiver system is tuned tothe microwave frequency and feeds a suitable detector circuit [56]References Cited responsive to the low frequency signal. A coincidenceUNITED STATES PATENTS circuit energizes an alarm circuit whenever thede- 3.7()7 7ll 12/1972 Cole eta]. 340/280 tected signal coincides withthe original modulation 3,7,848 l/l973 M3116 S 340/280 env glope beingto the low frequency genera 3,740.742 6/1973 Thompson et al. 340/258 C3.806.905 4/1974 Strenglcin 343/68 R 13 Claims, 5 Drawing Figures 3.55966 -,Qumuvs 32 MK Bums/e f= /ooHz Essa 5715p 2/455 4/10 X Couvrfie 2$700,059

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S'riEET 4 BMNWWVWWVWVWWWMMM/WVWWWMM/UM/UMM c PL FL H 11 EILIII'LH FUL irIr 11 L111 LI U11 1m SURVEILLANCE SYSTEM AND METHOD UTILIZING BOTHELECTROSTATIC AND ELECTROMAGNETIC FIELDS BACKGROUND OF THE INVENTION Thepresent invention relates to a method and appa ratus for pilferagecontrol. More particularly. it is directed to a method and apparatus fordetecting the presence of a telltale element in an unauthorized zone.

For the purpose of controlling pilferage. it has been proposedheretofore to secure specially constructed tags to the merchandise whichtags must be deactivated or removed for authorized removal of themerchandise from the controlled area. In one known arrangement, the tagsare provided with electrically resonant circuits embedded therein whichserve to dctune the tank circuit of an electronic oscillator whenbrought in proximity thereto for triggering an alarm. Use has also beenmade of tags incorporating a non-linear device in conjunction with anantenna element for reradiating the second harmonic ofa microwave signalwhich has been directed into the controlled space. Detection of saidsecond harmonic signal has been used to trigger an alarm However. theseknown methods have various limitations on their reliability andsensitivity. They are often susceptible to false triggering by metallicstructures coincidentally manifesting similar properties to the specialtags. Proximity of the human body to the electromagnetic fieldgenerating equipment or to the tags tends to mask the effect of theequipment and to interfere with reliable operation.

For a long time it has been known that a non-linear device will functionas a signal mixer producing sum and difference frequencies when excitedby two signals of differing frequencies. It has been suggested.heretofore. to establish two low frequency electromagnetic fields ofslightly different frequency within a space to be supervised and todetect the presence of signals corresponding to the difference betweensaid two frequencies. In this manner it is asserted to be possible todetect the presence ofa non-linear device within the controlled region.However. such system has many shortcomings not the least of which is thecost of producing a circuit ofpractical small size that can beincorporated in a tag and which is resonant at the low frequency.

It has also been suggested that the frequency of one of the twoelectromagnetic fields be chosen in the microwave region. While thisavoids some of the prob lems with the tag encountered when both fieldsare of low frequency. a different disadvantage exists. the microwaveenergy produces reflections and standing waves in the vicinity of thespace being supervised. This coupled with the increased propagatingcharacteristics of such high frequency energy results in considerableoverrange and false triggering of the surveillance system by tagsoutside of the intended controlled space.

SUMMARY OF THE INVENTION With the foregoing in mind. the presentinvention has for its object to provide a method for detecting thepresence in a controlled space of a miniature passive electric signalreceptor-reradiator which is superior to any method heretofore known.

In accordance with one aspect of the present invention there is provideda method of detecting within a confined space an electric signalreceptor-reradiator which has signal-mixing capability. the methodcomprising the steps of simultaneously establishing in the controlledspace first and second energy fields. The first energy field is chosento be electromagnetic in nature and is produced by a continuousmicrowave signal for causing the receptor-reradiator to return a signaltherefrom. The second field is chosen to be electrostatic in natureestablished by applying signal voltage to a discontinuous conductorrelative to a point of reference potential and having a sufficiently lowfrequency to enable it to be confined substantially to the controlledspace. Detection in the space of a signal consisting of a carrier andmodulation components where the components are due respectively to saidfirst and second fields is indicative of the presence of thereceptorreradiator therein.

In accordance with another aspect of the present invention. there isprovided a surveillance system for detecting the presence in acontrolled space of receptorreradiator receptor-reradiator of theforegoing type. said system comprising in combination a source ofcontinuous microwave signals. means coupled to the source of microwavesignals for propagating through said space an electromagnetic wavecorresponding to the microwave signals. a source of low frequencysignals. a discontinuous conductor coupled to the source of lowfrequency signals for establishing through the space an electrostaticfield corresponding to the low frequency signals, signal detectingmeans, means for coupling the detecting means with the space forreceiving signals therefrom the detecting means being constructed andarranged to detect the low frequency sig nals only when received asmodulation on a carrier signal whose frequency bears a predeterminedrelationship to that of the microwave signals. and means cou pled to thedetecting means for providing an alarm responsive to the detection ofthe low frequency signals.

DESCRIPTION OF THE DRAWINGS The invention will be better understoodafter reading the following detailed description of the presentlypreferred embodiments thereof with reference to the appended drawings inwhich:

FIG. I is a block diagram of one surveillance system constructed inaccordance with the invention;

FIG. 2 is a series of curves showing the signal waveforms at variouslocations identified by the corresponding reference letters in thesystem of FIG. 1;

FIG. 3 is a schematic diagram of a typical diodedipolereceptor-reradiator with signal-mixing capability;

FIG. 4 is a block diagram of another embodiment of the presentinvention; and

FIG. 5 is a series of curves showing the waveforms at various locationsidentified by the corresponding reference letters in the system of FIG.4.

DETAILED DESCRIPTION The same reference numerals are used throughout thevarious figures of the drawings to designate the same or similar parts.

Referring now to FIG. I, an ultrahigh frequency transmitter 10 operatingat 9l5 MHz functions as a source of microwave signals and has its outputconnected over path 11 to one input ofa circulator 12. The circulator I2passes the signal from the source 10 to a path 13 leading to asplitter-combiner (tce) element 14. The splitter-combiner element I4divides the signals received from the circulator into two componentswhich it feeds along the paths l5 and lb to the respective microwaveantenna elements 17 and 18. The antenna elements l7 and I8 may belocated in respective pedestals or enclosures represented symbolicallyby the phantom line boxes 19 and 20.

Any signals that might be received by the antenna elements l7 and 18from the adjacent space are fed back through the path and 16 to thesplitter-combiner l4 whereupon they are combined and fed through path 13back to the circulator 12. Such signals are then passed by thecirculator 12 to the path 21 leading to the first detector 22.

The nature of the circulator I2 is such that while most of the signalfrom the source 10 is fed from the path II to the path 13. some leakagedocs feed through from path 11 to path 21. This leakage component of themicrowave signal from the transmitter 10 is utilized in the firstdetector 22 for a purpose which will be described more fullyhereinafter.

The output of the first detector is fed over a path 23 through abandpass filter 24 to an AM detector 25 via a path 26. The output fromthe AM detector 25 is fed over a path 27 to one input of an AND circuit28. At the same time. the signal on path 27 is fed through a NOT orinverting circuit 29 to one input of a second AND circuit 30.

A free-running multivibrator operating at 100 Hz and designated by thereference numeral 31 has its output fed over a path 32 through a bufferamplifier 33 to a path 34 leading to the input to a pulse-controlled lowfrequency generator 35. The output of the generator 35 is fed over apath 36 to a discontinuous conductor 37 for a purpose to be described.In the present embodiment. the generator 35 operates at a frequency of22 KHZ.

The signal output from the multivibrator 31 on path 32 is also fed inparallel to the second input of each of the AND circuits 28 and 30. Theoutput from AND circuit 28 is fed over path 38 to the input ofa stepcounter 39 whose output is directed over path 40 to an input of pulsestretcher 41. The output from pulse stretcher 41 is directed over path42 to an alarm device 43. A reset signal for step counter 39 is derivedover path 44 from the output of AND circuit 30.

Assuming that the system of FIG. 1 is to be used'for controlling theegress from a retail store or the like. the two antenna elements 17 and18 would be mounted on either side of the exit doorway so as to producean electromagnetic field in the space therebetween. Preferably. theelements 17 and 18 have a radiation pattern generally confined to thespace to be controlled. The conductor 37 may be extended across theareaway so as to establish an electrostatic field throughout the samespace when energized by the output from pulsecontrolled generator 35relative to a point of reference potential. A grounded conductor (notshown) may be located in the floor in order to provide a return path forthe signals. if necessary. and to establish said point of referencepotential.

It has been discovered that if a small non-linear device in the form ofa semi-conductor rectifier chip or the like is connected to dipoleantenna elements of the proper length the device will function as asignal mixing circuit. taking the signals corresponding to both themicrowave transmitter and the low frequency pulse generator andmodulating the latter signal upon the former for reradiation. Such adevice. i.e.. a diode-dipole. is shown schematically in FIG. 3 with thenon-linear device or rectifier 45 connected to dipole elements 46 and 47which are all embedded in a tag 48. Lumped capacitance and inductanceelements are not needed. For efficient operation at 915 MHz. thetip-to-tip length of the elements 45. 46 and 47 should theoretically beof the order of l6.4 cm. In practice slight departure from thetheoretical value may be found beneficial By appropriate folding of theends of the dipole elements back upon themselves, it is possible toincorporate the structure in a smaller overall configuration.

Referring to FIG. 2. the output of free-running multivibrator 31 is aseries of square pulses represented by curve A thereof. These pulseshave a duration of approximately 2 milliseconds. In the present example.the repetition rate is Hz. The pulses from multivibrator 31, afterpassing through the buffer 33, function to turn on the generator 35 soas to provide frequency or pulse bursts therefrom. This is representedby curve B in FIG. 2. Thus. assuming the presence of a tag 48 in thespace between the antenna elements 17 and 18, a signal will bereradiated back to the elements 17 and 18 consisting of a carriercomponent at 9l5 MHz modulated by square wave bursts ofa 22 KHZmodulating signal. That is. the signal returned to splitter-combiner 14will have a fundamental component at 915 MHZ plus sum and differencefrequencies equal to 915.022 and 914.978 MHz. These signals join withthe leakage signal at 9 l 5 MHZ fed over path 21 to the first detector22. The detector 22 may include a rectifier mixer for eliminating thecarrier frequency, i.e.. the 9l5 MHz component. In cooperation with thebandpass filter 24 which has a center frequency of 22 KHz and abandwidth of 2 KHZ there is derived a signal on path 26 having afrequency of 22 KHz and corresponding to the 22 KHz component present inthe modulated signal intercepted by antenna elements 17 and 18.

Still assuming that the tag 48 is in the space being supervised. thesignal fed to the AM detector 25 over path 26 will duplicate the outputof generator 35 and have the form shown in curve B of FIG. 2. The AMdetector 25 may be any conventional detector. capable of producing a DC.output proportional to the amplitude of the input signal. Where theinput signal has the form shown in curve B of FIG. 2. the output of thedetector 25 will be as shown in curve C of FIG. 2.

So long as the signals fed over path 27 from the detector 25 to the ANDcircuit 28 coincide with the output from the multivibrator 31represented by curve A of FIG. 2, a pulse will be developed at theoutput of the AND circuit 28 fed to the step counter 39 and countedtherby. If the preceding condition prevails for a period of timesufficient to enable the step counter to reach its preset capacity. anoutput pulse will be fed to the pulse stretcher 4] for energizing thealarm 43. At present. it is preferred that the step counter provide anoutput after 16 input pulses. Any other count may be employed asdesired.

Because of the inversion caused by the circuit 29, the AND circuit 30will not produce an output signal so long as the signals on paths 32 and27 are similar and coincident. However, as soon as the signals providedby the AM detector 25 disappear. a reset pulse will be furnished by ANDcircuit 30 to reset the step counter 39. This will occur in any eventwhen the tag 48 is removed from the controlled space.

If. however. the antenna elements [7 and l8 pick up a signal due to someartifact. it is not likely that such artifact will produce a sequence of16 properly shaped and timed pulses. If as shown at point 49 on curve Cof FIG. 2 there is no signal received. a reset pulse 50 as shown incurve D of FIGv 2 will be applied to the counter 39. If a broken pulse51 as shown in curve C is provided by the detector 25. then the resetpulse 52 as shown in curve D will be applied to the counter 39. Itshould therefore be apparent that either through the absence of a returnpulse or the reception ofa defective reutrn pulse the counter will bereset to commence another count anew.

On the other hand, once a full count is received so as to activate thepulse stretcher 41, it is possible for the step counter to be reset afew times before another valid count is received without interruptingthe alarm. By properly relating the time duration of the pulse stretcher4] and the number of counts required by the counter 39. it is possibleto optimize the response of the overall system for distinguishingbetween valid signals and artifact.

While the system shown in FIG. I is quite effective. increasedsensitivity and selectivity is provided by the system now to bedescribed with reference to FIG. 4 to which attention should bedirected. As shown therein, the ultrahigh frequency transmitter 55 hasits power output fed over path 56 through a 3db isolator pad 57 and abandpass filter 58 to the splitter 59. The bandpass filter 58 has acenter frequency of9l5 MHz. The splitter 59 has two outputs connectedover paths 60 and 61 to individual antenna elements 62 and 63,respectively. The antenna elements 62 and 63 should be mounted onopposite sides of the area to be controlled in corresponding enclosuresor pedestals such as those represented by the broken line boxes 64 and65. In this manner, the two antenna elements 62 and 63 establish anelectromagnetic field of microwave energy in the controlled spacctherebetween.

A second pair of antenna elements 66 and 67 are mounted across thecontrolled space from the corresponding transmitter antenna elements 62and 63, respectively. The signals received from the space by antennaelements 66 and 67 are fed over corresponding paths to the two inputs ofa combiner element 68 whose common output is fed over path 69 through abandpass filter 70 to one input of a balanced mixer 71. The second inputof the balanced mixer 71 is furnished with a signal at 915 MHz derivedfrom a low power level output of the transmitter 55 over path 72. Thebandpass filter 70 has a center frequency of 9i 5 MHz.

The output from the balanced mixer 71 is fed over path 73 to the FMdetector 74 whose output is fed to the input of a NAND gate 75. Theoutput from NAND gate 75 is fed over one path to one input of NAND gate76 and over another path to the input of NAND gate 77. The output ofNAND gate 77 is fed to one input of NAND gate 78. The output of NANDgate 76 is fed to the input ofa l6-count counter 79 whose output isconnected through a pulse stretcher 80 to an alarm circuit 81. The resetterminal of counter 79 is connected to the output of NAND gate 78.

A voltage-controlled multivibrator pulse generator 82 operating atselectable rates between 200 and 250 Hz has its output connected over apath 83 to an attenuator 84 whose output is fed to the controlling inputof a voltage-controlled multivibrator pulse generator 85.

The generator 85 has a center frequency of 50 KHz. In response to thepulse control received through attenuator 84 from generator 82 thefrequency of generator 85 is shifted il KHz between 49 KHz and 51 KHz.The output from generator 85 is fed through a resonant type low passfilter 86 whereby the square wave is converted to a sinusoidal signal oflike frequency which is fed over path 87 to a power amplifier 88. Theoutput of the power amplifier is connected over separate paths 89 and 90to corresponding step-up transformers 9l and 92. The secondary windings(not shown) of the transformers 9i and 92 are connected to apply voltageto the foil elements 93 and 94 associated. respectively. with each ofthe housings 64 and 65. The foils constitute a special form ofdiscontinuous conductor. The signals fed to the foils 93 and 94 are inparallel and establish an electrostatic field between the respectivefoils and ground, i.e.. the point of reference potential. Effectiveresults have been obtained across an 8 ft. space with foils or platesmeasuring no more than about 4 inches X 4 inches and excited by a signalof about 245 V. RMS. Both the energizing voltage and the foil size maybe varied depending upon the area to be supervised.

A second path 95 conducts the output of the generator 82 through abuffer amplifier 96 to a delay multivibrator 97. The output of the dalaymultivibrator 97 is fed over path 98 to the input of a reference pulsemultivibrator 99 whose output is fed over path 100 to the second inputof each of the NAND gates 76 and 78.

The operation of the circuit of FIG. 4 will now be described withreference to the waveforms shown in the various curves of Flg. 5 whereinthe letters appended to the individual curves correspond to the lettersappearing on FIG. 4. A selection switch (not shown) may be used toselect the desired pulse rate for generator 82. For example. theselectable rates may be 200. 225 and 250 Hz. At present. the preferredrate is 200 Hz al though rates between lOO and 500 Hz have been usedexperimentally. The generator 82 provides a symmetrical square waveoutput as shown in curve A of FIG. 5. This signal is reduced by theattenuator 84 to the proper level for shifting the frequency of pulsegenerator 85 between 49 KHz and SI KHz. While the output of generator 85is square wave in nature, it is converted by the resonant type low passfilter 86 to a sinusoidal signal as represented in curve B of FIG. 5.Such signal is then amplified by the power amplifier 88 and employed todrive the foils 93 and 94 for creating the electrostatic field in thecontrolled space.

As in the embodiment of FIG. 1, when a receptorradiator such as shown inFIG. 3 is introduced into the controlled space a modulated signal isdeveloped which will be received by antenna receiving elements 66 and 67and applied to the balanced mixer 71. In known manner. the balancedmixer 71 will remove the 9l5 MHz carrier frequency component and supplythe 49 KHz and 51 KHz as detected thereby over path 73 to the FMdetector 74 for conversion to a square wave pulse having the form shownin curve E in FIG. 5. It should be understood that detector 74 may be aconventional ratio detector or the like. Preferably, the input to thedetector 74 is provided with a high gain amplifier-limiter (not shown)while the output of the detector 74 includes a low pass filter (notshown) to ensure removal of the 49-51 KHz component.

Curve C of FIG. shows the pulse output provided by delay multivibrator97. It will be seen that the leading edge of the pulse produced bymultivibrator 97 coincides with the leading edge of the positive goingpulse output of generator 82 shown in curve A. The trailing edge of thepulse produced by multivibrator 97 may be adjustable by appropriatemeans not shown. The delay produced by multivibrator 97 is therebyadjusted to be equal to the normal delay encountered by the signals inpassing through the equipment both into the electrostatic field and backon the modulated carrier through the balanced mixing and detectingcircuitry.

The trailing edge of the pulse from multivibrator 97 is employed totrigger the reference pulse multivibrator 99 whose output is shown bycurve D. The leading edge of the pulse produced by multivibrator 99coincides with the trailing edge of the pulse output ofmultivibrator 97.The width of the pulse produced by multivibrator 99 may be adjustable bymeans not shown so that such pulse width coincides with the pulse widthreceived from an actual tag in the controlled space.

Bearing in mind that the FM detector 74 is arranged to produce a DC.signal of one level in response to a 51 KHZ input signal and a DC.signal ofa second level in response to a 49 KHz signal, it will beappreciated that the output derived from the NAND gate 75 will have theform shown in curve E when a tag 48 is in the controlled space. Thesubsequent operation of the system is quite similar to that previouslydescribed with reference to FIG. 1. When the signal at the output of theNAND gate 75 has a waveform or envelope which coincides with the signalat the output of multivibrator 99 on path I00 represented by curve D apulse count will be introduced into the counter 79. After [6 suchcounts, an output will be fed to the pulse stretcher 80 to energize thealarm 81. Any suitable counter may be employed for this purpose. Forexample, use may be made of a tandem arrangement of four Jl( flip-flopsconnected in known manner to produce an output at the completion of acount of sixteen.

Should the pulse output from detector 74 be interrupted due to removalof the tag from the controlled space or due to some other interferencethe counter 79 will be reset in a manner whick should be evident fromthe previous discussion and from the drawings.

By way of further explanation, curve E shows a possible artifact typesignal which might result from extra neous factors. It should bementioned that the counter 79 is actuated both as to its input and resetterminals by negative going pulses. Input pulses ol'the type shown incurve E will cause count pulses of the type shown in curve Finterspersed with reset pulses as shown in curve G'. Consequently, thecounter will be repeatedly reset and will not reach its output count.

It should now be appreciated that with either the circuit of FIG. 1 orFIG. 4, an alarm will be given only when the detected low frequencysignal has a wave envelope coinciding with an output of the pulsemodulating means. In the system of FIG. I, the multivibrator 3]represents the pulse modulating means, while in the system of FIG. 4 thepulse modulating means is represented by generator 82. This concept isextended in the system of FIG. 4 wherein an alarm is given only when thedetected low frequency signal is frequency modu' lated with a waveenvelope having the same shape as the modulating signal.

In both systems the low frequency signal should be preferably no greaterthan [00 KHz. It has been found that such low frequencies are bestemployed for establishing the electrostatic field. On the other hand.the high frequency signal should be in the microwave region so that thephysical size of the tags need not be excessive.

Having described the presently preferred embodi ments of the inventionit should be understood that various changes in construction andarrangement will be apparent to those skilled in the art and are fullycontemplated herein without departing from the true spirit of theinvention as defined in the appended claims.

What is claimed is:

l. The method of maintaining surveillance within a confined space todetect the presence in said space of an electric signalreceptor-reradiator with signal mixing capability. said methodcomprising the steps of simulataneously establishing in said space firstand second energy fields, said first field being electromagnetic innature and produced by a continuous microwave signal for causing saidreceptor-reradiator to return a signal therefrom, said second fieldbeing electrostatic in nature established by applying a signal voltageto a discontinuous conductor relative to a point of reference potentialand having a sufficiently low frequency to enable it to be confinedsubstantially to said space. and detecting the presence in said space ofa signal consist ing of a carrier and modulation components where saidcomponents are due respectively to said first and second fields.

2. The method according to claim 1, wherein said second field isproduced with a frequency modulated signal.

3. A surveillance system for detecting the presence in a controlledspace of a minature passive electromagnetic wave receptor-reradiatorwith signal mixing capability, said system comprising in combination asource of continuous microwave signals, means coupled to said source ofmicrowave signals for propagating through said space an electromagneticwave corresponding to said microwave signals, a source of low frequencysignals, a discontinuous conductor coupled to said establishing of lowfrequency signals for extablishing through said space an electrostaticfield corresponding to said low frequency signals, said low fre' quencysignals having a sufficiently low frequency to enable said electrostaticfield to be confined substantially to said space, signal detectingmeans, means for coupling said detecting means with said space forreceiving signals therefrom, said detecting means being constructed andarranged to detect said low frequency signals only when received asmodulation on a carrier signal whose frequency bears a predeterminedrelationship to that of said microwave signals, and means coupled tosaid detecting means for providing an alarm responsive to detection ofsaid low frequency signals.

4. A surveillance system according to claim 3, wherein saiddiscontinuous conductor comprises a plate-like member.

5. a surveillance system for detecting the presence in a controlledspace of a miniature passive diode-dipole electromagnetic wavereceptor-reradiator with signal mixing capability, said systemcomprising in combination a source of microwave signals, means coupledto said source of microwave signals for propagating through said spacean electromagnetic wave corresponding to said microwave signals. asource of low frequency signals. a discontinuous conductor coupled tosaid source of low frequency signals for establishing through said spacean electrostatic field corresponding to said low frequency signals. saidlow frequency signals having a sufficiently low frequency to enable saidelectrostatic field to be confined substantially to said space. signaldetecting means. means for coupling said detecting means with said spacefor receiving signals therefrom. said detecting means being constructedand arranged to detect said low frequency signals only when received asmodulation on a carrier signal having the same frequency as saidmicrowave signals. and means coupled to said detecting means forproviding an alarm responsive to detection of said low frequencysignals.

6. A surveillance system according to claim 5, wherein means are coupledto said source of low frequency signals for pulse modulating the latter.and wherein said means for providing an alarm are coupled to said pulsemodulating means for providing said alarm only when the detected lowfrequency signal has a wave envelope coinciding with an output of saidpulse modulating means.

7. A surveillance system for detecting the presence in a controlledspace of a miniature passive electromagnetic wave rcceptor-reradiatorwith signal mixing capability. said system comprising in combination asource of microwave signals. means coupled to said source of microwavesignals for propagating through said space an electromagnetic wavecorresponding to said microwave signals. a source of low frequencysignals, means coupled to said source of low frequency signals for frequency modulating the latter with a modulating signal. further meanscoupled to said source of low frequency signals for establishing throughsaid space an electrostatic ficld corresponding to said low frequencysignals, said low frequency signals having a sufficiently low frequencyto enable said electrostatic field to be confined substantially to saidspace. signal detecting means. means for coupling said detecting meanswith said space for receiving signals therefrom. said detecting meansbeing constructed and arranged to detect said low frequency signals onlywhen received as modulation on a carrier signal whose frequency bears apredetermined relationship to that of said microwave signals. and meanscoupled to said detecting means for providing an alarm responsive todetection of said low frequency signals.

8. A surveillance system according to claim 7. wherein said means forproviding an alarm are coupled to said frequency moduating means forproviding said alarm only when the detected low frequency signals arefrequency modulated with a wave envelope having the same shape as saidmodulating signal.

9. A surveillance system according to claim 8. wherein said modulatingsignal has the form ofa square wave.

10. A surveillance system according to claim 7. wherein said modulatingsignal has the form ofa square wave.

11. A surveillance system according to claim 7. wherein said frequencymodulation of said source of low frequency signals is characterized by afrequancy deviation of the order of lKHz.

l2. A surveillance system for detecting the presence in a controlledspace ofa miniature passive electromagnetic wave receptor-reradiatorwith signal mixing capability; said system comprising in combination asource of microwave signals; means coupled to said source of microwavesignals for propagating through said space an electromagnetic wavecorresponding to said microwave signals: a source of low frequencysignals; means coupled to said source of low frequency signals forestablishing through said space an electrostatic field corresponding tosaid low frequency signals; said low frequency signals having asufficiently low frequency to enable said electrostatic field to beconfined substantially to said space; said source of low frequencysignals comprising a voltage-controlled multivibrator pulse generator.means coupled to an output of said pulse generator for converting asquare wave signal to a sinusoidal signal for use in establishing saidelectrostatic field, and means coupled to said pulse generator forfrequency modulating the latter with a modulating signal; signaldetecting means; means for coupling said detecting means with said spacefor receiving signals therefrom. said detecting means being constructedand arranged to detect said low frequency signals only when received asmodulation on a carrier signal whose frequency bears a predeterminedrelationship to that of said microwave signals; and means coupled tosaid detecting means for providing an alarm responsive to detection ofsaid low frequency signals.

13. A surveillance system according to claim 12, wherein said modulatingsignal has the form ofa square wave.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION PATENT NO.3,895,368 DATED y 15, 1975 |NVENTOR(S) Lloyd L, Gordon et a1 It iscertifred that error appears in the above-identified patent and thatsaid Letters Patent are hereby corrected as shown below:

Column 1, line 51, "the" should read --The--.

Colunm 2, lines 18 and 19, "receptor-reradiator receptor-reradiator"should read --a receptorreradiator--; line 30, after "therefrom" inserta comma Column 5, line 12, "reutrn" should read -return-.

Column 6 line 27, "dalay" should read --delay--; line 34, "FIg." shouldread --FIG.--.

Column 7, line 45, "whick" should read -which--.

Column 8, lines 18 and 19, "simula-taneously should read--simul-taneously-; line 44, "establishing" should read --source--;lines 44 and 45, "extablish-ing" should read --establish-ing--; line 61,"a" should read -A--.

Signed and Sealed this fourth Day Of November I 975 [SEAL] A nest:

RUTH C. MASON C. MARSHALL DANN Arresting Officer Commissioner oflaremsand Tradenmrlu-

1. The method of maintaining surveillance within a confined space todetect the presence in said space of an electric signalreceptor-reradiator with signal mixing capability, said methodcomprising the steps of simulataneously establishing in said space firstand second energy fields, said first field being electromagnetic innature and produced by a continuous microwave signal for causing saidreceptor-reradiator to return a signal therefrom, said second fieldbeing electrostatic in nature established by applying a signal voltageto a discontinuous conductor relative to a point of reference potentialand having a sufficiently low frequency to enable it to be confinedsubstantially to said space, and detecting the presence in said space ofa signal consisting of a carrier and modulation components where saidcomponents are due respectively to said first and second fields.
 2. Themethod according to claim 1, wherein said second field is produced witha frequency modulated signal.
 3. A surveillance system for detecting thepresence in a controlled space of a minature passive electromagneticwave receptor-reradiator with signal mixing capability, said systemcomprising in combination a source of continuous microwave signals,means coupled to said source of microwave signals For propagatingthrough said space an electromagnetic wave corresponding to saidmicrowave signals, a source of low frequency signals, a discontinuousconductor coupled to said establishing of low frequency signals forextablishing through said space an electrostatic field corresponding tosaid low frequency signals, said low frequency signals having asufficiently low frequency to enable said electrostatic field to beconfined substantially to said space, signal detecting means, means forcoupling said detecting means with said space for receiving signalstherefrom, said detecting means being constructed and arranged to detectsaid low frequency signals only when received as modulation on a carriersignal whose frequency bears a predetermined relationship to that ofsaid microwave signals, and means coupled to said detecting means forproviding an alarm responsive to detection of said low frequencysignals.
 4. A surveillance system according to claim 3, wherein saiddiscontinuous conductor comprises a plate-like member.
 5. a surveillancesystem for detecting the presence in a controlled space of a miniaturepassive diode-dipole electromagnetic wave receptor-reradiator withsignal mixing capability, said system comprising in combination a sourceof microwave signals, means coupled to said source of microwave signalsfor propagating through said space an electromagnetic wave correspondingto said microwave signals, a source of low frequency signals, adiscontinuous conductor coupled to said source of low frequency signalsfor establishing through said space an electrostatic field correspondingto said low frequency signals, said low frequency signals having asufficiently low frequency to enable said electrostatic field to beconfined substantially to said space, signal detecting means, means forcoupling said detecting means with said space for receiving signalstherefrom, said detecting means being constructed and arranged to detectsaid low frequency signals only when received as modulation on a carriersignal having the same frequency as said microwave signals, and meanscoupled to said detecting means for providing an alarm responsive todetection of said low frequency signals.
 6. A surveillance systemaccording to claim 5, wherein means are coupled to said source of lowfrequency signals for pulse modulating the latter, and wherein saidmeans for providing an alarm are coupled to said pulse modulating meansfor providing said alarm only when the detected low frequency signal hasa wave envelope coinciding with an output of said pulse modulatingmeans.
 7. A surveillance system for detecting the presence in acontrolled space of a miniature passive electromagnetic wavereceptor-reradiator with signal mixing capability, said systemcomprising in combination a source of microwave signals, means coupledto said source of microwave signals for propagating through said spacean electromagnetic wave corresponding to said microwave signals, asource of low frequency signals, means coupled to said source of lowfrequency signals for frequency modulating the latter with a modulatingsignal, further means coupled to said source of low frequency signalsfor establishing through said space an electrostatic field correspondingto said low frequency signals, said low frequency signals having asufficiently low frequency to enable said electrostatic field to beconfined substantially to said space, signal detecting means, means forcoupling said detecting means with said space for receiving signalstherefrom, said detecting means being constructed and arranged to detectsaid low frequency signals only when received as modulation on a carriersignal whose frequency bears a predetermined relationship to that ofsaid microwave signals, and means coupled to said detecting means forproviding an alarm responsive to detection of said low frequencysignals.
 8. A surveillance system according to claim 7, wherein saidmeans for providing an alarm are coupled to said frequencY moduatingmeans for providing said alarm only when the detected low frequencysignals are frequency modulated with a wave envelope having the sameshape as said modulating signal.
 9. A surveillance system according toclaim 8, wherein said modulating signal has the form of a square wave.10. A surveillance system according to claim 7, wherein said modulatingsignal has the form of a square wave.
 11. A surveillance systemaccording to claim 7, wherein said frequency modulation of said sourceof low frequency signals is characterized by a frequancy deviation ofthe order of 1KHz.
 12. A surveillance system for detecting the presencein a controlled space of a miniature passive electromagnetic wavereceptor-reradiator with signal mixing capability; said systemcomprising in combination a source of microwave signals; means coupledto said source of microwave signals for propagating through said spacean electromagnetic wave corresponding to said microwave signals; asource of low frequency signals; means coupled to said source of lowfrequency signals for establishing through said space an electrostaticfield corresponding to said low frequency signals; said low frequencysignals having a sufficiently low frequency to enable said electrostaticfield to be confined substantially to said space; said source of lowfrequency signals comprising a voltage-controlled multivibrator pulsegenerator, means coupled to an output of said pulse generator forconverting a square wave signal to a sinusoidal signal for use inestablishing said electrostatic field, and means coupled to said pulsegenerator for frequency modulating the latter with a modulating signal;signal detecting means; means for coupling said detecting means withsaid space for receiving signals therefrom, said detecting means beingconstructed and arranged to detect said low frequency signals only whenreceived as modulation on a carrier signal whose frequency bears apredetermined relationship to that of said microwave signals; and meanscoupled to said detecting means for providing an alarm responsive todetection of said low frequency signals.
 13. A surveillance systemaccording to claim 12, wherein said modulating signal has the form of asquare wave.